1. Field of the Invention
The present invention relates to a code division multiple access (CDMA) mobile phone in a digital communication system, and more particularly a circuit for minimizing or eliminating external interference signals in such a mobile phone.
2. Description of the Related Art
Generally, mobile phone or cellular communication systems employ multiple access communication techniques such as CDMA or Advanced Mobile Phone Service (AMPS). Typically, digital systems employ CDMA and frequency modulation is used in AMPS. CDMA may be used in dual mode to allow reception of an analog signal. Namely, it may be used not only in CDMA mode but also in AMPS mode. In such usage, when digital signals are communicated, there often occur analog interfering signals to cut a call signal or cause erroneous reception of the digital signals. This error is called intermodulation distortion (IMD), which affects a CDMA mobile phone near an AMPS radio station. A strong external interference signal overloads the low noise amplifier of the CDMA phone so that the IMD component severely interferes with the CDMA frequency band, causing it to be off-line. Following is a specific description of the influence of such interference signals.
The received interference signals pass through the low noise amplifier to produce second and third order harmonics Eo, which may be expressed by Eq. 1. EQU Eo=k.sub.1 Ei+k.sub.2 Ei.sup.2 +k.sub.3 Ei.sup.3 Equation 1!
If there are generated dual tones with the same amplitude and different frequencies, the dual tones are expressed by Eq. 2. EQU Ei=A(cos.omega..sub.1 t+cos.omega..sub.2 t) Equation 2!
Substituting Ei of equation 2 in equation 1: ##EQU1##
As expressed by Eq. 3, two signals with different frequencies F1 and F2 are amplified to generate non-linear components in the form of the products obtained by multiplying the mixed forms of the second and third orders. The second order mixing is 2F1, 2F2, F1+F2 and F1-F2. Such signals of the mixed component occur outside the narrow band of most systems, and therefore are negligible. The third order mixing is 2F1+F2, 2F1-F2, 2F2-F1 and 2F2+F1, whose signals are IMDs occurring in the pass band of most systems.